Plot hole: When they find captain Nemo's journal the journal is completely intact and the pages are made of paper. But the journal is very old, and the island sinks every 70 years, so the journal should not have looked that good, and the pages must be gone by now or at least not that readable. (00:47:30)
Plot hole: As the island is sinking you can see parts break off, like the island is falling apart, but if it does this every 70 years then there wouldn't be an island. It should have completely fallen apart hundreds of years ago.
Answer: There is a deleted scene on the DVD/Bluray where they attempt to give an explanation. Kailani asks "What happens to the animals when the island sinks? They can't all die!". Alexander replies "Even the mammals develop quasi-amphibian characteristics. They just burrow below the ground's surface and absorb the oxygen through their skin."
Sierra1 ★